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Effect of Elevated Carbon Dioxide Concentration on Rice Quality: Proximate Composition, Dietary Fibers, and Free Sugars

¹CITAB – Centre for the Research and Technology of Agro-Environment and Biological Sciences, Universidade de Trás-os-Montes e Alto Douro, 5001-801 Vila Real, Portugal. ²CECAV, Universidade de Trás-os-Montes e Alto Douro, 5001-801 Vila Real, Portugal. ³Instituto Nacional de Investigação Agrária e Veterinária, Av. República, Nova Oeiras, 2784-505 Oeiras, Portugal. ⁴Corresponding author. Phone: +351 259 350 751. Fax: +351 259 350 327. E-mail: htrindad@utad.pt

Little is known regarding the impact of elevated carbon dioxide concentration ([CO₂]) on the chemical composition of rice grains. A field experiment was conducted with open-top chambers with rice (Oryza sativa L. cv. Ariete) grown at two levels of atmospheric CO₂ (375 and 550 μmol/mol), and their effects were monitored on the proximate composition and carbohydrate contents of the grains. Following exposure to elevated [CO₂], soluble dietary fiber increased by 136, 82, and 77% in brown rice, white rice, and bran, respectively. Increases of a lower magnitude (8%) were observed for insoluble dietary fiber in the bran and brown rice. For all 10 sugars identified, there was a trend for increasing their content. For example, increases of 135% were recorded for glucose in the white rice. In all rice milling fractions, elevated [CO₂] reduced the protein (4–15%) and amylose (6–16%) contents, with no effect on the ash, starch, and gross energy contents. The fat content was increased by elevated [CO₂] in the white rice (23%) and tended to decrease in the bran (9%). It is concluded that besides yield, increased dietary fiber might be another positive effect of high levels of atmospheric CO₂ expected by the middle of the current century.